The present development is a high efficiency catalyst for use in a catalytic partial oxidation process for the production of hydrogen or syngas gas from hydrocarbons. The catalyst comprises rhenium in combination with a second metal selected from the group consisting of platinum, iridium, ruthenium, rhodium, and palladium at an atomic ratio of rhenium to second metal of from 25:1 to 1:1. The process comprises reacting a feed containing hydrocarbons with an oxygen source at a C/O ratio of about 0.9 to about 1.1 in the presence of the catalyst, and wherein the gas hourly space velocity of the feed over the catalyst ranges from about 1,000 hr−1 to about 2,000,000 hr−1. In the process, the catalyst is maintained at a temperature of from about 500° C. to about 1,500° C. as the feed makes contact with the catalyst.
Large volumes of hydrogen or synthesis gas, a mixture of hydrogen and carbon monoxide, are needed for a number of important chemical reactions. One process for producing hydrogen or synthesis gas is through catalytic partial oxidation (COPx) processes. In catalytic partial oxidation processes the gaseous hydrocarbon feedstock is mixed with air, oxygen-enriched air, or oxygen, in the presence of a catalyst. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 1:CH4+½O2CO+2H2  (1)This product ratio is especially desirable for downstream applications such as the Fischer-Tropsch Synthesis.
In a catalytic partial oxidation process, the following side-reactions may occur:CH4+2O2CO2+2H2O  (2)CO+H2OCO2+H2  (3)The reaction shown in Equation 2 reduces the selectivity of both hydrogen and carbon monoxide. When the reaction occurs, CO2 and H2O are formed. The system then favors the reaction shown in Equation 3, which alters the H2CO ratio (the ratio is greater than 2:1 in the forward direction and less than 2:1 in the reverse direction). Catalysts for partial oxidation processes reported in the prior art tend to favor production of gases with a H2/CO ratio of less than 2:1 because the catalyst has <100% selectivity toward Equation 1 and high reaction temperatures needed for efficient methane conversion favor the reverse reaction of Equation 3. With a high efficiency catalyst, the reactions of Equation 2 and Equation 3 can be minimized or even totally eliminated.
In catalytic partial oxidation processes, the feedstock is normally introduced to the catalyst at elevated temperature and pressure. For example, in a typical catalytic partial oxidation process, the feedstock is preheated to a temperature of about 450° C., the gas hourly space velocity of the feed over the catalyst ranges from about 1,000 hr−1 to about 2,000,000 hr−1, the pressure in the reactor can be up to 300 atms, and the catalyst is maintained at a temperature of from about 500° C. to about 1,500° C. as the feed makes contact with the catalyst. The reaction generally proceeds at a relatively fast rate, and shorter catalyst contact times are needed to accomplish partial oxidation of a hydrocarbon feedstock as compared to prior art steam reforming processes.
The selectivity of catalytic partial oxidation to produce carbon monoxide and hydrogen is controlled by several factors—one of the most important being the choice of catalyst composition. Typically, the best catalyst compositions include precious metals. Rhodium, for example, has been found to be effective in CPOx processes, but rhodium also tends to have a lower stability than desired for commercial applications. Rhodium loading at concentrations of from about 3 wt % to about 10 wt % has been found to reduce the catalyst deactivation rate. In addition, to achieve high selectivity to the target products, carbon monoxide and hydrogen, and to produce synthesis gas with a H2/CO ratio as close to 2.0 as possible, a temperature higher than 1000° C. is general required for a rhodium catalyst.
U.S. Pat. No. 5,648,582 (issued to Schmidt, et al. on Jul. 15, 1997) teaches a CPOx process having a very short residence time (120,000 to 12,000,000 hr−1 at atmospheric pressure) that uses a ceramic monolith supporting a metal catalyst selected from rhodium, nickel or platinum. U.S. Pat. No. 6,458,334 (issued to Tamhankar, et al on Oct. 1, 2002) teaches a CPOx process initiated at temperatures below 200° C. and having a relatively short residence time (50,000 hr−1 to 12,000,000 hr−1 at a pressure of 1 to 20 atmospheres) that uses a ceria monolith supporting a metal catalyst selected from nickel, cobalt, iron, platinum, palladium, iridium, rhenium, ruthenium, rhodium, osmium and combinations thereof. The difficulty with using a monolith support is that the catalyst must be engineered to fit the reactor, which is operated at an elevated temperature. Because of the difference in expansion characteristics between the monolith and the reactor, it is possible for feed to bypass the catalyst. In addition, it can be difficult to find coating materials with the stability and durability needed for high temperature processes in large-scale applications.
For the CPOX process to be of commercial interest, it is necessary that the catalyst used in the process is able to achieve a high conversion of the methane feedstock at high gas hourly space velocities (GHSV) for a long period of time, and that the selectivity of the process to the desired products, carbon monoxide and hydrogen, must be as high as possible. Moreover, the catalyst preferably should comprise metals that are readily available at reasonable costs.